Typical Magnetoresistive random access memory (MRAM) structures have a nonmagnetic layer sandwiched between two ferromagnetic films. The two ferromagnetic films are also known as magnetic thin films. The MRAM employs the magneto resistive properties of this structure to store data. In each storage element, an MRAM employs two lines, commonly termed a word line and a sense string, in order to detect the magnetization direction of these magnetic thin films. Each string comprises a magnetic thin film that serves as a memory element, and the word line generally addresses multiple sense strings. Magnetic thin films that have a parallel moment have a low resistance and are typically assigned the ‘1’ state. Magnetic thin films having an anti-parallel moment have a high resistance and are typically assigned the ‘0’ state, but may also be assigned to the ‘1’ state.
During a read operation, a word current passes through the word line causing the magnetic layers in the sense string to rotate, thereby changing the resistance in the sense string. A sense current passes through the sense string. A sense line receives the signal from the sense string. A differential amplifier compares the signal from the sense line to a reference line to determine whether a one resistance or a zero resistance is stored in the MRAM. A differential amplifier notes the change in voltage across the sense line to determine resistive state of a storage element.
MRAM operation requiring only small variations from the desired operation conditions is preferred. Changes, such as normal variances, induced during the fabrication process may require different operational biases and associated references than the default circuitry provides.
Others have tried having one setting or a sampling of chips with different settings and then testing for all good or bad chips. Methods of changing analog operating points, such as laser trimming, are well known but have not been utilized for MRAM manufacture.
There is a need to compensate for these differences in operating conditions.
There is a further need for a method to change the default setting level of the bias circuitry to match the required operation point.
There is a further need to optimize circuit performance of sensitive circuits.
There is a further need to provide a way of testing and setting analog operating points after chip fabrication of the MRAM.